Static Scanning Tunneling Microscopy Images Reveal the Mechanism of Supramolecular Polymerization of an Oligopyridine on Graphite

Abstract Supramolecular polymerization of a donor–acceptor bisterpyridine (BTP) equipped with an electron‐rich carbazole unit is observed by scanning tunneling microscopy (STM) at the highly oriented pyrolytic graphite (HOPG)|solution interface. It is shown that two‐dimensional crystals of supramolecular (co)polymers are formed by chain growth polymerization, which in turn can be described by copolymerization statistics. From concentration‐dependent measurements, derived copolymerization parameters and DFT calculations, a mechanism for self‐assembly is developed that suggests a kinetically driven polymerization process in combination with thermodynamically controlled crystallization.


DLS
All DLS measurements were conducted using a Malvern Panalytical Zetasizer Nano ZS. Several measurements with different concentrations of 2,2'-BTPCz in 1,2,4-trichlorobenzene were performed to exclude possible aggregate formation. The curves in Figure S 2 display the average of several measurements. It can be seen that the mean size is about 0.7 nm for both concentrations, which indicates that there are no aggregates present. Although sub-nanometre sized particles are at the limits of the instrument's performance, the absence of readings above 1 nm means that the formation of aggregates can be excluded. The classic Arrhenius approach delivers the following equations assuming that the preexponential factors are equal since there is only one type of reactants: 5 mg/mL 2.5 mg/mL Between 1800 and 7800 monomeric and oligomeric sequences of configuration B (between 2500 and 11500 molecules) were counted manually from minimum three images or domains for each concentration ( Figure S 2 -Figure S 4). Contrasts in the images which could not unambiguously assigned to a specific sequence were omitted. The corresponding numbers are shown in Table S 1. The errors were determined from the number fractions of the sequences in every image or domain, respectively. In addition, the rows with the respective sequences were counted independently. If a jump from one sequence to another occurred within a row, each sequence block was counted as a separate row.

Simulation of the number fractions N B (n) and determination of the copolymerization parameters r
For the determination of the copolymerization parameters " and " ′ according to the penultimate model the following equations according to the literature were applied. [3] " ( ) = A fit procedure of the experimental data for ≥ 2 with Eq. 1 and " and " ′ as fitting parameters was carried out with Origin. [4] " ′ was also determined from Eq. 2. For the simulation of " ( ) (see Figure 4) the weighted number average of " ′ from Eq. 1 and 2 was taken.
The incorporated amounts of A and B in the copolymer FA and FB and the average sequence lengths nB(av) were calculated according to Eq. 3 to 5 with the help of an Excel program provided by the literature. [3] Please note that these simplified equations are only valid for equal feed ratios fA = fB = 0.5 (equal probability of both configurations A and B) and rA = rA' = 0 (no "reaction" between chain end A and monomer A, see Scheme S 2).

Computational details
The calculations were carried out using the xTB program package (6.4.0) from Grimme's group. [5] The xTB GFN code uses an extended semiempirical tight-binding model parameterized to obtain good values for geometry, forces and nonbonding interactions. The energies were calculated with GFN2-xTB. [6] Some preoptimizations were carried out using the gfn-ff force field provided by recent versions of xTB. [7] A single graphene layer consisting of 592 C-atoms terminated with 74 H-atoms (26 x 10 C6rings) was used as substrate. The graphene layer was optimized in advance. The positions of its atoms were constrained during the subsequent calculations. A trimer on the graphene layer with two p overlaps was optimized. In the starting structure the molecule on the righthand side was adsorbed in a planar way, while in the two others the carbazole unit was twisted in order to allow for p overlaps. During optimization the left molecule slips from the middle molecule and adsorbs in a planar way on the substrate whereas the middle molecule still stays overlapping with the right one ( Figure S 11). In order to achieve a planar structure for this middle molecule, one of the two molecules situated to the left or to the right from it would have to move by a considerable distance. Apparently, the lateral shift of a molecule adsorbed fully on the graphene has a high activation energy. This supports the dominating kinetic control of the self-assembled structures.

Estimation of adsorption energy
The adsorption energy of a single BTP molecule on the graphene layer was determined to -399 kJ mol -1 . The lateral displacement of a molecule with a protruding carbazole unit due to p overlap (configuration B) to a vdW-bound and fully planar adsorbed molecule (configuration A) amounts to around 0.4 nm. Based on the unit cell parameter of the dimer along the oligomer axis (4.16 nm, see Table 1 in main text), this means an additional increase of the unit cell by around 10%. Thus, the gain in adsorption energy of the reduced space due to the p overlap is -40 kJ mol -1 . Compared with the endothermic cost of the p overlap (+22 kJ mol -1 ), the total gain of the p overlap is -18 kJ mol -1 .